1. Field of the Invention
This invention relates to an improved catalyst useful in the conversion of hydrocarbons, particularly for the isomerization of alkanes.
2. General Background
The isomerization of light naphtha is an increasingly important process for the upgrading of petroleum refiners' gasoline pool. The widespread removal of lead antiknock additive from gasoline and the rising demands of high-performance internal-combustion engines are increasing the need for "octane," or knock resistance, in the gasoline pool. In the early years of lead removal, refiners relied principally upon increasing the octane of products from catalytic reforming and fluid catalytic cracking operations. Refiners have largely capitalized on these relatively low-cost upgrading options, however, and attention has turned in recent years to upgrading the relatively low-octane high-naphtha component.
Catalyst and process technology for the isomerization of light alkanes is well known in the art. The recent expansion of interest, however, has led to significant improvements in this technology. Catalyst and process developments have led to lower operating temperatures, wherein product octane is favored by isomer equilibrium. Substantial reduction in the hydrogen requirement for a stable operation has resulted in a significant cost reduction, principally through elimination of the need for a hydrogen-recycle system. Both of the aforementioned developments have led toward a predominance of liquid in the isomerization reactor feed, in contrast to the vapor-phase operation of the prior art.
Catalysts exhibiting a dual cracking and hydrogenation-dehydrogenation function are applied widely in the petroleum refining and petrochemical industries to the reforming and isomerization of hydrocarbons. Such catalysts generally have the cracking function imparted by an inorganic oxide, zeolite, or halogen, with a platinum-group component usually imparting the hydrogenation-dehydrogenation function. A catalyst useful in isomerization must be formulated to properly balance its cracking and hydrogenation-dehydrogenation functions to achieve the desired conversion over a prolonged period of time, while effectively utilizing the expensive platinum group metal component.
The performance of a catalyst in isomerization service typically is measured by its activity, selectivity, and stability. Activity refers to the ability of a catalyst to isomerize the reactants into the desired product isomers at a specified set of reaction conditions. Selectivity refers to the proportion of converted feed reacted into the desired product. Stability refers to the rate of change of activity and selectivity during the life of the catalyst. The principal cause of low catalyst stability is the formation of coke, a high-molecular-weight, hydrogen-deficient, carbonaceous material on the catalytic surface. Workers in the isomerization art thus must address the problem of developing catalysts having high activity and stability, and which also either suppress the formation of coke or are not severely affected by the presence of coke.
Currently, workers in the art are faced by the problem of developing cost-effective catalysts meeting the above objectives as a concomitant to the low isomerization temperatures and high proportion of liquid reactants described hereinabove.
3. Related Art
Catalysts containing a platinum-group metal component and a Friedel-Crafts metal halide on a refractory inorganic-oxide binder are known in the art. For example, U.S. Pat. No. 2,900,425 (Bloch et al.) teaches an isomerization process characterized by a catalyst comprising alumina, a platinum-group metal, and a Friedel-Crafts metal halide. Preferably, aluminum chloride is sublimed on to the alumina-platinum composite. Bloch et al. do not disclose a surface-layer platinum-group metal; however, the alternative methods of compositing the platinum-group metal and alumina are all effected before formation of the catalyst particle. See also U.S. Pat. Nos. 2,924,629 (Donaldson), teaching an isomerization process; 2,999,074 (Bloch et al.), teaching a catalyst composition of matter; and 3,031,419, teaching a method for manufacturing a catalyst, none of which disclose a surface-layer platinum-group metal component.
U.S. Pat. No. 3,963,643 (Germanas et al.) teaches a method of manufacturing a catalyst useful in the isomerization of paraffins by compositing a platinum-group metal with alumina and reacting the composite with a Friedel-Crafts metal halide and a polyhalo compound. Germanas et al. teaches away from a surface-layer platinum-group metal, noting that: "It is common practice to impregnate the alumina with an aqueous chloroplatinic acid solution acidified with hydrochloric acid to facilitate an even distribution of platinum on the alumina . . . " (col. 2, lines 49-53).
The layering of a platinum-group metal component in a catalyst particle has been disclosed in the prior art. U.S. Pat. Nos. 3,259,589 (Michalko), 3,388,077 (Hoekstra) and 3,931,054 (Lester) teach a catalyst preparation method for providing a subsurface-layer of Group VIII metal or platinum in a catalyst particle. U.S. Pat. No. 3,367,888 (Hoekstra) discloses a method of catalyst preparation wherein the Group VIII metal is deposited on the outer surface of the carrier. U.S. Pat. No. 3,651,167 (deRosset) teaches a hydrogenation process characterized by a catalyst comprising a surface-impregnated Group VIII metal or alumina. U.S. Pat. Nos. 3,897,368 (Ohara) and 4,431,750 (McGinnis) disclose catalyst preparation methods wherein a noble or platinum-group metal is deposited in high concentration on the surface of the support. U.S. Pat. Nos. 4,520,223 teaches a method of preparation of a surface-impregnated noble metal catalyst useful in a dehydrogenation process. U.S. Pat. Nos. 4,716,143 (Imai) and 4,786,625 (Imai et al.) disclose catalysts comprising surface-impregnated platinum, but teach that the catalysts decrease undesirable side reactions such as isomerization. U.S. Pat. No. 4,556,646 (Bezman) teaches a catalyst with an even radial distribution of noble metal, and reveals that low penetration of Pd into a catalyst base increases coking in a hydrocracking reaction. Further, none of these patents disclose the essential Friedel-Crafts metal halide component of the present catalyst.
Thus, no suggestion is offered, in well over 20 years of prior art pertaining to individual components of the present catalyst particle, to combine a Friedel-Crafts metal halide and a surface-layer platinum-group metal component on a refractory inorganic-oxide support. In conformity with the unpredictability of catalytic effects, the surprising benefits of this catalyst particle are observed specifically in the context of modern, primarily liquid-phase, isomerization operations.